This issue has been reported by Jan Hjelmager.
Draft 6 of the GeoDCAT-AP specification in its objectives (section 1.1), explains that the objective of the GeoDCAT-AP specs is to "define an RDF syntax that can be used for the exchange of descriptions of spatial datasets among data portals".
It is questioned whether it is only for datasets or also for series and services as described in clause 1 of the introduction.
Editorial issue. "dataset series, and services" was added to the sentence.
"The objective of this work is to define an RDF syntax that can be used for the exchange of descriptions of spatial datasets, dataset series, and services among data portals."
I think that a "dataset series" or for that matter any broader logical collection of individual datasets (from one publisher) is again a dataset. So you might want to have DCAT metadata for the whole series but also for each individual dataset. The missing link is how to create the parent-child relation. My suggestion: https://schema.org/hasPart and https://schema.org/isPartOf.
EDIT: In fact, I just found dct:hasPart and dct:isPartOf which are more appropriate.